The most fundamental property of biological membranes is to serve as a selective barrier, allowing the penetration of only solutes of certain classes. The structural basis of these functions will be investigated by using several experimental systems from bacteria. (1) The outer membrane, located outside the peptidoglycan layer and the cytoplasmic membrane of gram-negative bacteria, is an ideal model membrane for the study of this type, because its functions are very simple in that it allows mainly passive and facilitated diffusion processes. The diffusion of hydrophilic solutes are mediated by porin and other specific channels, and the properties of these channels will be characterized. Areas that will be emphasized will include the voltage- and pressure-mediated closing of the porin channel, the identity and properties of porin channels in Pseudomonas aeruginosa, and the functional architecture of specific channels such as the phage lambda receptor (maltoporin) channel. In addition, unusual specific transport systems that require the collaboration of TonB protein will be studied by using a newly developed assay. Finally, the molecular basis of the unusually low permeability of lipid bilayer region of the outer membrane will be studied by utilizing intact cells, planar bilayers, and bilayer vesicles. The results of these studies are of great medical interest, as most of the antibiotic-resistant bacterial pathogens causing hospital-acquired infections are bacteria covered with outer membranes of low permeability. They can thus suggest ways to produce more effective antibiotics that can overcome this barrier. (2) The mycobacterial cell wall is rich in lipidic constituents, and was recently shown to act as an extremely effective permeability barrier. We will study how hydrophilic molecules diffuse across this barrier. If porin-like proteins can be identified and characterized, this will again suggest ways of improving the penetration of antibiotics and chemotherapeutic agents into these bacteria, especially "atypical" mycobacteria well-known for their antibiotic resistance and their capability of causing intractable secondary infections in many AIDS patients. (3) The molecular mechanism of transport of maltose across the cytoplasmic membrane of Escherichia coli will be studied. This system is of interest not only because it is a highly complex and efficient transport machinery, but also its component proteins share a strong sequence homology with the P-glycoprotein that pumps out anti-cancer drugs from some of the tumor cells.